No doubt...this whole thing stinks like 4 day old fish. I think these two folks knew each other...well. And it went south in a major way.
I’m thinking that, too.
Two possibilities:
1. They had some form of relationship and out of all other possible arrangements, he just happened to have an apartment directly above hers.
2. She didn't know him and really did go to the wrong floor by mistake and entered his apartment because it was directly above hers.
#1 is very unlikely
#2 is the more simple and likely explanation, particularly if she was impaired in some way.